U.S. patent number 10,080,891 [Application Number 14/957,651] was granted by the patent office on 2018-09-25 for extra-cardiovascular cardiac pacing system.
This patent grant is currently assigned to Medtronic, Inc.. The grantee listed for this patent is Medtronic, Inc.. Invention is credited to Melissa G. T. Christie, Mark T. Marshall, Thomas H. Spear, Amy E. Thompson-Nauman.
United States Patent |
10,080,891 |
Thompson-Nauman , et
al. |
September 25, 2018 |
Extra-cardiovascular cardiac pacing system
Abstract
An extra-cardiovascular medical device is configured to select a
capacitor configuration from a capacitor array and deliver a low
voltage, pacing pulse by discharging the selected capacitor
configuration across an extra-cardiovascular pacing electrode
vector. In some examples, the medical device is configured to
determine the capacitor configuration based on a measured impedance
of the extra-cardiovascular pacing electrode vector.
Inventors: |
Thompson-Nauman; Amy E. (Ham
Lake, MN), Christie; Melissa G. T. (Ham Lake, MN),
Marshall; Mark T. (Forest Lake, MN), Spear; Thomas H.
(Bloomington, MN) |
Applicant: |
Name |
City |
State |
Country |
Type |
Medtronic, Inc. |
Minneapolis |
MN |
US |
|
|
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
|
Family
ID: |
57281301 |
Appl.
No.: |
14/957,651 |
Filed: |
December 3, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170157395 A1 |
Jun 8, 2017 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61N
1/3925 (20130101); A61N 1/3956 (20130101); A61N
1/36521 (20130101); A61N 1/3706 (20130101) |
Current International
Class: |
A61N
1/36 (20060101); A61N 1/08 (20060101); A61N
1/37 (20060101); A61N 1/365 (20060101); A61N
1/39 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2004050178 |
|
Jun 2004 |
|
WO |
|
2004052444 |
|
Jun 2004 |
|
WO |
|
Other References
(PCT/US2016/059632) PCT Notification of Transmittal of the
International Search Report and the Written Opinion of the
International Searching Authority, dated Feb. 2, 2017, 11 pages.
cited by applicant .
Marshall et al., "Substernal Lead/Electrode Concepts", U.S. Appl.
No. 62/089,417, filed Dec. 9, 2014, 28 pages. cited by
applicant.
|
Primary Examiner: Evanisko; George
Claims
The invention claimed is:
1. An extra-cardiovascular medical device, comprising: an impedance
measurement module configured to measure an impedance of an
extra-cardiovascular pacing electrode vector when
extra-cardiovascular pacing electrodes are electrically coupled to
the medical device; a therapy delivery module comprising a
capacitor array having a plurality of capacitors for producing a
pacing pulse; and a pacing control module coupled to the therapy
delivery module and the impedance measurement module and configured
to: control the impedance measurement module to measure the
impedance of the extra-cardiovascular pacing electrode vector;
determine a first capacitor configuration based on the measured
impedance by determining a capacitance required for the measured
impedance to produce an RC time constant that is greater than a
time interval threshold and identifying the first capacitor
configuration so that the first capacitance is equal to or greater
than the determined capacitance, the first capacitor configuration
comprising a first combination of the plurality of capacitors
having a first capacitance; control the therapy delivery module to
select the first capacitor configuration by selectively enabling
the first combination of the plurality of capacitors of the
capacitor array; and control the therapy delivery module to deliver
a cardiac pacing pulse by discharging the first capacitor
configuration for a predetermined pulse width across the
extra-cardiovascular pacing electrode vector.
2. The device of claim 1, wherein the time interval threshold is
greater than the pacing pulse width.
3. The device of claim 1, wherein: the therapy delivery module is
configured to sample the amplitude of the pacing pulse during a
first portion of the pulse width and during a second portion of the
pulse width after the first portion; and the pacing control module
is further configured to: receive the sampled pacing pulse
amplitude from the therapy delivery module; compare the sampled
pacing pulse amplitude to a first threshold during the first
portion of the pulse width; in response to the sampled pacing pulse
amplitude being greater than the first threshold, continue
delivering the pacing pulse using the first capacitor
configuration; compare the sampled pacing pulse amplitude to a
second threshold during the second portion of the pulse width, the
second threshold lower than the first threshold; determine a second
capacitor configuration in response to the pacing pulse amplitude
being less than or equal to the second threshold during the second
portion of the pulse width, the second capacitor configuration
comprising a second combination of the plurality of capacitors of
the capacitor array; control the therapy delivery module to select
the second capacitor configuration by selectively enabling the
second combination of the plurality of capacitors of the capacitor
array; and control the therapy delivery module to continue
delivering the pacing pulse by discharging the second capacitor
configuration during the second portion of the pulse width.
4. The device of claim 1, wherein: the therapy delivery module is
configured to sample the amplitude of the pacing pulse; and the
pacing control module is further configured to: receive the sampled
pacing pulse amplitude from the therapy delivery module; compare
the sampled pacing pulse amplitude to an amplitude threshold; in
response to the sampled pacing pulse amplitude being greater than
the amplitude threshold, adjust the first capacitor configuration
to a second combination of the plurality of capacitors of the
capacitor array, the second combination having a second capacitance
less than the first capacitance; control the therapy delivery
module to enable the adjusted first capacitor configuration by
selectively enabling the second combination of the plurality of
capacitors of the capacitor array; and control the therapy delivery
module to deliver a next pacing pulse by discharging the adjusted
first capacitor configuration for the predetermined pulse
width.
5. The device of claim 1, further comprising a memory storing a
plurality of impedance ranges and a plurality of capacitor
configurations, wherein one of the plurality of capacitor
configurations is stored for each one of the plurality of impedance
ranges, wherein the pacing control module is configured to
determine the first capacitor configuration by matching the
measured impedance to one of the plurality of impedance ranges and
selecting the capacitor configuration stored for the matching
impedance range.
6. An extra-cardiovascular medical device comprising: an impedance
measurement module configured to measure an impedance of an
extra-cardiovascular pacing electrode vector when
extra-cardiovascular pacing electrodes are electrically coupled to
the medical device; a therapy delivery module comprising a
capacitor array having a plurality of capacitors for producing a
pacing pulse; and a pacing control module coupled to the therapy
delivery module and the impedance measurement module and configured
to: control the impedance measurement module to measure the
impedance of the extra-cardiovascular pacing electrode vector;
determine a first capacitor configuration based on the measured
impedance, the first capacitor configuration comprising a first
combination of the plurality of capacitors having a first
capacitance; control the therapy delivery module to select the
first capacitor configuration by selectively enabling the first
combination of the plurality of capacitors of the capacitor array;
and control the therapy delivery module to deliver a cardiac pacing
pulse by discharging the first capacitor configuration for a
predetermined pulse width across the extra-cardiovascular pacing
electrode vector, wherein the therapy delivery module is configured
to sample the amplitude of the pacing pulse after a first portion
of the pulse width; and the pacing control module is further
configured to: receive the sampled pacing pulse amplitude from the
therapy delivery module; compare the sampled pacing pulse amplitude
to a first amplitude threshold; determine a second capacitor
configuration in response to the sampled pacing pulse amplitude
being less than or equal to the first amplitude threshold; control
the therapy delivery module to select the second capacitor
configuration by selectively enabling the second combination of the
plurality of capacitors of the capacitor array; and control the
therapy delivery module to continue delivering the pacing pulse by
discharging the second capacitor configuration during a second
portion of the pulse width, the second portion after the first
portion.
7. The device of claim 6, wherein the second capacitor
configuration is selected to have a second capacitance that is
greater than the first capacitance.
8. The device of claim 6, wherein the pacing control module is
configured to set the first amplitude threshold to a percentage of
a leading edge amplitude of the pacing pulse.
9. The device of claim 6, wherein the second capacitor
configuration comprises the first capacitor configuration and at
least one capacitor of the plurality of capacitors that was not
included in the first capacitor configuration.
10. The device of claim 6, wherein: the therapy delivery module is
configured to sample the amplitude of the pacing pulse during the
second portion of the pulse width; and the pacing control module is
further configured to: compare the amplitude of the pacing pulse
during the second portion of the pulse width to a second amplitude
threshold; and control the impedance measurement module to repeat
the measurement of the impedance of the pacing electrode vector in
response to the amplitude of the pacing pulse being less than the
second amplitude threshold during the second portion of the pulse
width.
11. A method performed by an extra-cardiovascular medical device,
comprising: measuring an impedance of an extra-cardiovascular
pacing electrode vector when extra-cardiovascular pacing electrodes
are electrically coupled to the medical device; determining by a
pacing control module of the medical device a first capacitor
configuration based on the measured impedance by determining a
capacitance required for the measured impedance to produce an RC
time constant that is greater than a time interval threshold and
identifying the first capacitor configuration so that the first
capacitance is equal to or greater than the determined capacitance,
the first capacitor configuration comprising a first combination of
a plurality of capacitors of a capacitor array; selecting the first
capacitor configuration by selectively enabling the first
combination of the plurality of capacitors; and delivering a
cardiac pacing pulse by discharging the first capacitor
configuration for a predetermined pulse width across the
extra-cardiovascular pacing electrode vector.
12. The method of claim 11, wherein the time interval threshold is
greater than the pacing pulse width.
13. The method of claim 11, further comprising sampling the
amplitude of the pacing pulse during a first portion of the pulse
width and during a second portion of the pulse width after the
first portion; receiving the sampled pacing pulse amplitude from
the therapy delivery module; comparing the sampled pacing pulse
amplitude to a first threshold during the first portion of the
pulse width; in response to the sampled pacing pulse amplitude
being greater than the first threshold, continuing delivering the
pacing pulse using the first capacitor configuration; comparing the
sampled pacing pulse amplitude to a second threshold during the
second portion of the pulse width, the second threshold lower than
the first threshold; determining a second capacitor configuration
in response to the pacing pulse amplitude being less than or equal
to the second threshold during the second portion of the pulse
width, the second capacitor configuration comprising a second
combination of the plurality of capacitors of the capacitor array
and having a second capacitance greater than the first capacitance;
selecting the second capacitor configuration by selectively
enabling the second combination of the plurality of capacitors of
the capacitor array; and continuing delivering the pacing pulse by
discharging the second capacitor configuration during the second
portion of the pulse width.
14. The method of claim 11, further comprising: sampling the
amplitude of the pacing pulse; comparing the sampled pacing pulse
amplitude to an amplitude threshold; in response to the sampled
pacing pulse amplitude being greater than the amplitude threshold,
adjusting the first capacitor configuration to a second combination
of the plurality of capacitors of the capacitor array, the second
combination having a second capacitance less than the first
capacitance; enabling the adjusted first capacitor configuration by
selectively enabling the second combination of the plurality of
capacitors of the capacitor array; and delivering a next pacing
pulse by discharging the adjusted first capacitor configuration for
the predetermined pulse width.
15. The method of claim 11, further comprising: storing in a memory
of the medical device a plurality of impedance ranges and a
plurality of capacitor configurations, wherein one of the plurality
of capacitor configurations is stored for each one of the plurality
of impedance ranges; and determining the first capacitor
configuration by matching the measured impedance to one of the
plurality of impedance ranges and selecting the capacitor
configuration stored for the matching impedance range.
16. A method comprising: measuring an impedance of an
extra-cardiovascular pacing electrode vector when
extra-cardiovascular pacing electrodes are electrically coupled to
the medical device; determining by a pacing control module of the
medical device a first capacitor configuration based on the
measured impedance by determining a capacitance required for the
measured impedance to produce an RC time constant that is greater
than a time interval threshold and identifying the first capacitor
configuration so that the first capacitance is equal to or greater
than the determined capacitance, the first capacitor configuration
comprising a first combination of a plurality of capacitors of a
capacitor array; selecting the first capacitor configuration by
selectively enabling the first combination of the plurality of
capacitors; and delivering a cardiac pacing pulse by discharging
the first capacitor configuration for a predetermined pulse width
across the extra-cardiovascular pacing electrode vector; sampling
the amplitude of the pacing pulse after a first portion of the
pulse width; comparing the sampled pacing pulse amplitude to a
first amplitude threshold; determining a second capacitor
configuration in response to the sampled pacing pulse amplitude
being less than or equal to the first amplitude threshold;
selecting the second capacitor configuration by selectively
enabling the second combination of the plurality of capacitors of
the capacitor array; and continuing delivering the pacing pulse by
discharging the second capacitor configuration during a second
portion of the pulse width, the second portion after the first
portion.
17. The method of claim 16, wherein determining the second
capacitor configuration comprises determining a second capacitance
of the second capacitor configuration that is greater than the
first capacitance of the first capacitor configuration.
18. The method of claim 16, further comprising setting the first
amplitude threshold to a percentage of a leading edge amplitude of
the pacing pulse.
19. The method of claim 16, further comprising selecting the second
capacitor configuration by selecting the first capacitor
configuration and at least one capacitor of the plurality of
capacitors that was not included in the first capacitor
configuration.
20. The method of claim 16, further comprising: sampling the
amplitude of the pacing pulse during the second portion of the
pulse width; comparing the amplitude of the pacing pulse during the
second portion of the pulse width to a second amplitude threshold;
and repeating the impedance measurement of the pacing electrode
vector in response to the amplitude of the pacing pulse being less
than the second amplitude threshold during the second portion of
the pulse width.
Description
TECHNICAL FIELD
The disclosure relates generally to implantable medical devices
and, in particular, to a system, device and method for delivering
cardiac pacing pulses using extra-cardiovascular electrodes.
BACKGROUND
A variety of implantable medical devices (IMDs) for delivering a
therapy, monitoring a physiological condition of a patient or a
combination thereof have been clinically implanted or proposed for
clinical implantation in patients. Some IMDs employ one or more
elongated electrical leads carrying stimulation electrodes, sense
electrodes, and/or other sensors. IMDs may deliver therapy to or
monitor conditions of a variety of organs, nerves, muscle or
tissue, such as the heart, brain, stomach, spinal cord, pelvic
floor, or the like. Implantable medical leads may be configured to
position electrodes or other sensors at desired locations for
delivery of electrical stimulation or sensing of physiological
conditions. For example, electrodes or sensors may be carried along
a distal portion of a lead that is extended subcutaneously,
transvenously, or submuscularly. A proximal portion of the lead may
be coupled to an implantable medical device housing, which contains
circuitry such as signal generation circuitry and/or sensing
circuitry.
Some IMDs, such as cardiac pacemakers or implantable cardioverter
defibrillators (ICDs), provide therapeutic electrical stimulation
to the heart of the patient via electrodes carried by one or more
implantable leads and/or the housing of the pacemaker or ICD. The
leads may be transvenous, e.g., advanced into the heart through one
or more veins to position endocardial electrodes in intimate
contact with the heart tissue. Other leads may be non-transvenous
leads implanted outside the heart, e.g., implanted epicardially,
pericardially, or subcutaneously. The electrodes are used to
deliver electrical stimulation pulses to the heart to address
abnormal cardiac rhythms.
IMDs capable of delivering electrical stimulation for treating
abnormal cardiac rhythms typically sense signals representative of
intrinsic depolarizations of the heart and analyze the sensed
signals to identify the abnormal rhythms. Upon detection of an
abnormal rhythm, the device may deliver an appropriate electrical
stimulation therapy to restore a more normal rhythm. For example, a
pacemaker or ICD may deliver low voltage pacing pulses to the heart
upon detecting bradycardia or tachycardia using endocardial or
epicardial electrodes. An ICD may deliver high voltage
cardioversion or defibrillation shocks to the heart upon detecting
fast ventricular tachycardia or fibrillation using electrodes
carried by transvenous leads or non-transvenous leads. The type of
therapy delivered and its effectiveness in restoring a normal
rhythm depends at least in part on the type of electrodes used to
deliver the electrical stimulation and their location relative to
heart tissue.
SUMMARY
In general, the disclosure is directed to techniques for delivering
extra-cardiovascular cardiac pacing pulses. A pacemaker or ICD
operating according to the techniques disclosed herein measures a
pacing electrode vector impedance and determines a capacitor
configuration based on the measured impedance and pacing pulse
duration. The capacitor configuration is selected so that an RC
time constant of the discharging capacitor configuration across the
pacing electrode vector results in a pacing pulse having a
truncated pulse amplitude that is greater than a threshold
amplitude. Extra-cardiovascular pacing may be delivered at a pacing
pulse amplitude below a pain threshold of the patient with a pulse
width long enough to deliver adequate energy to successfully pace
the heart.
In one example, the disclosure provides an extra-cardiovascular
medical device including an impedance measurement module configured
to measure an impedance of an extra-cardiovascular pacing electrode
vector when extra-cardiovascular pacing electrodes are electrically
coupled to the medical device. The medical device includes a
therapy delivery module having a capacitor array for producing a
pacing pulse and a pacing control module coupled to the therapy
delivery module and the impedance measurement module. The pacing
control module is configured to control the impedance measurement
module to measure the impedance of the extra-cardiovascular pacing
electrode vector, determine a capacitor configuration based on the
measured impedance, and control the therapy delivery module to
deliver a cardiac pacing pulse by discharging the capacitor
configuration for a predetermined pulse width across the
extra-cardiovascular pacing electrode vector.
In another example, the disclosure provides a method performed by
an extra-cardiovascular medical device including measuring an
impedance of an extra-cardiovascular pacing electrode vector when
extra-cardiovascular pacing electrodes are electrically coupled to
the medical device, determining a capacitor configuration based on
the measured impedance, and delivering a cardiac pacing pulse by
discharging the first capacitor configuration for a predetermined
pulse width across the extra-cardiovascular pacing electrode
vector.
In another example, the disclosure provides a non-transitory,
computer-readable storage medium comprising a set of instructions
which, when executed by a control module of an extra-cardiovascular
medical device, cause the medical device to measure an impedance of
an extra-cardiovascular pacing electrode vector when
extra-cardiovascular pacing electrodes are electrically coupled to
the medical device, determine a capacitor configuration based on
the measured impedance, and deliver a cardiac pacing pulse by
discharging the capacitor configuration for a predetermined pulse
width across the extra-cardiovascular pacing electrode vector.
In another example, the disclosure provides an extra-cardiovascular
medical device including a therapy delivery module having a
capacitor array including a plurality of capacitors for producing a
pacing pulse and a pacing control module coupled to the therapy
delivery module. The pacing control module is configured to select
a first capacitor configuration comprising a first combination of
the plurality of capacitors and control the therapy delivery module
to select the first capacitor configuration by selectively enabling
the first combination of the plurality of capacitors of the
capacitor array. The pacing control module is configured to start a
pulse width timing interval and control the therapy delivery module
to start delivery of a cardiac pacing pulse by discharging the
first capacitor configuration across an extra-cardiovascular pacing
electrode vector coupled to the therapy delivery module. The pacing
control module is further configured to obtain a sampled amplitude
of the cardiac pacing pulse during the pulse width timing interval,
compare the sampled amplitude to an amplitude threshold, and
control the therapy delivery module to select a second capacitor
configuration comprising a second combination of the plurality of
capacitors different than the first combination and start
discharging the second capacitor configuration across the
extra-cardiovascular pacing electrode vector before the pulse width
timing interval expires in response to the amplitude of the cardiac
pacing pulse being less than or equal to the amplitude
threshold.
This summary is intended to provide an overview of the subject
matter described in this disclosure. It is not intended to provide
an exclusive or exhaustive explanation of the apparatus and methods
described in detail within the accompanying drawings and
description below. Further details of one or more examples are set
forth in the accompanying drawings and the description below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a conceptual diagram of a patient implanted with an
example extra-cardiovascular IMD system that includes a
subcutaneously implanted IMD coupled to an extra-cardiovascular
sensing, pacing and cardioversion/defibrillation (CV/DF) lead.
FIG. 2 is a transverse view of a patient depicting an alternative,
substernal location of the extra-cardiovascular lead of FIG. 1.
FIG. 3 is a schematic diagram of the IMD of FIG. 1 according to one
example.
FIG. 4 is a schematic diagram of a pacing control module and a
therapy delivery module included in IMD 14.
FIG. 5 is a schematic diagram of the pacing control module of FIG.
4.
FIG. 6 is a schematic diagram of a capacitor selection and control
module included in the therapy delivery module of FIG. 4.
FIG. 7A is a conceptual diagram of a pacing pulse generated by the
therapy delivery module of FIG. 4.
FIG. 7B is a conceptual diagram of a pacing pulse that may be
delivered by the IMD of FIG. 1 using a capacitor configuration that
is adjusted during the pacing pulse delivery.
FIG. 7C is a conceptual diagram of one example of a pacing pulse
that may be delivered by the IMD of FIG. 1 using an adjusted
capacitor configuration during the pacing pulse when a pacing
electrode vector impedance change occurs.
FIG. 8 is a conceptual diagram of a look-up table stored in memory
of the IMD of FIG. 1.
FIG. 9 is a flow chart of a method performed by the IMD of FIG. 1
for delivering a pacing pulse according to one example.
FIG. 10 is a flow chart that corresponds to operations performed in
the flow chart of FIG. 9 for delivering a pacing pulse.
FIG. 11 is a flow chart of a pacing control method according to
another example.
DETAILED DESCRIPTION
In general, this disclosure describes techniques for delivering low
voltage pacing pulses using extra-cardiovascular electrodes that
are not directly contacting cardiac tissue. As used herein, the
term "extra-cardiovascular" refers to a position outside the blood
vessels, heart, and pericardium surrounding the heart of a patient.
Implantable electrodes carried by extra-cardiovascular leads may be
positioned extra-thoracically (outside the ribcage and sternum) or
intra-thoracically (beneath the ribcage or sternum) but not in
intimate contact with myocardial tissue. The term "low voltage" in
reference to extra-cardiovascular pacing pulses refers to a voltage
level that is below a pain threshold of a patient and may be on the
order of 20 V or less in some examples.
Cardiac pacing is commonly delivered using electrodes in close or
intimate contact with myocardial tissue such as endocardial
electrodes or epicardial electrodes. Pacing pulses delivered using
endocardial electrodes are typically up to a maximum of 8 V in
pulse amplitude with a pulse width of 2.0 ms or less. A pacing
pulse that successfully paces the heart might be 2.5 V in amplitude
with a 0.5 ms pulse width, for example. The pulse amplitude and
pulse width are selected to provide a pacing pulse having adequate
energy to capture the heart, e.g., cause depolarization of the
ventricles of the heart.
ICD systems have been proposed or are commercially available that
utilize electrodes carried by subcutaneous or submuscular leads to
sense electrocardiogram (ECG) signals and deliver high-energy
shocks to cardiovert or defibrillate the heart. Electrodes carried
by subcutaneous or submuscular leads may be used for delivering
high-voltage, short-duration pulses during a post-shock recovery
phase to treat asystole that may occur post-shock. Discomfort
caused by these high voltage pacing pulses may be deemed acceptable
post-shock in light of the life-saving treatment provided by shock
delivery and post-shock, high-voltage pacing.
Such electrodes positioned subcutaneously or submuscularly are
generally not used for delivering pacing therapies that are
sustained over longer time periods or for conditions that are not
immediately life threatening, e.g., for bradycardia pacing,
anti-tachycardia pacing (ATP), or cardiac resynchronization therapy
(CRT). The relatively high voltage amplitude required to
successfully capture the heart when the electrodes are not in close
contact with the myocardium may cause intolerable or unacceptable
pain or discomfort to the patient. The high voltage is required in
order to deliver enough energy within a limited pacing pulse width,
e.g., 2 ms or less. This maximum pacing pulse width is limited by
the decay rate of the pacing pulse which is dependent on the
capacitance of the capacitor being discharged to deliver the pacing
pulse and the impedance of the pacing electrode vector through
which the capacitor is discharged
For a given pulse width typically used with endocardial or
epicardial electrodes, e.g., less than 2 ms, the pulse amplitude
required to capture the heart using the same pulse width when
pacing with extra-cardiovascular electrodes, such as subcutaneous
or submuscular electrodes, may cross an acceptable pain threshold.
A pacing pulse having a lower voltage amplitude that is not painful
to the patient when delivered by extra-cardiovascular electrodes
may require a relatively long pulse width that is likely to be
beyond the capacity of a typical low voltage pacing capacitor due
to the relatively fast decay rate of the pulse amplitude. A low
voltage pacing capacitor may have a capacitance of 10 microfarads
or less. Since a pacing pulse is delivered as the pacing capacitor
is discharged across the pacing electrode vector, the pacing pulse
amplitude may decay below an effective voltage amplitude before the
required pacing pulse width is reached for successfully delivering
the pacing pulse energy required to capture and pace the heart.
As disclosed herein, an implantable, extra-cardiovascular medical
device system includes a therapy delivery module having an array of
capacitors that are controlled by a pacing control module for
delivering low voltage pacing pulses having a pulse width that is
long enough, e.g., greater than 1.5 ms, to successfully pace the
heart using a pulse amplitude that is below a pain threshold of the
patient without requiring electrodes in direct contact with the
myocardial or pericardial tissue. The pacing control module
controls the decay rate of the pacing pulse by selecting a
capacitor configuration that maintains the amplitude of the pacing
pulse above a target amplitude for the duration of the pulse width.
The techniques disclosed herein may be implemented in any
implantable pacemaker or ICD and particularly in a pacemaker or ICD
having extra-cardiovascular electrodes. The electrodes may be
carried by a medical electrical lead extending from the pacemaker
or ICD and/or carried by the housing of the pacemaker or ICD. The
techniques disclosed herein are not necessarily limited to
implantable systems and may be implemented in an external pacemaker
or ICD using cutaneous surface electrodes or transcutaneous
electrodes.
FIG. 1 is a conceptual diagram of a patient 12 implanted with an
extra-cardiovascular IMD system 10 that includes a subcutaneously
implanted IMD 14 coupled to an extra-cardiovascular sensing, pacing
and cardioversion/defibrillation (CD/DF) lead 16. IMD 14 includes a
housing 15 and connector assembly 17. IMD 14 acquires cardiac
electrical signals, e.g., ECG signals, using electrodes carried by
lead 16 and may be configured to deliver pacing pulses using
extra-cardiovascular electrodes carried by lead 16. As will be
described herein, IMD 14 includes a pacing control module that
controls an array of pacing capacitors for delivering pacing pulses
via extra-cardiovascular electrodes. The pacing pulses have a pulse
amplitude that is less than the pain threshold of the patient and a
pulse width that is long enough, e.g., greater than 1.5 ms or
greater than 2.0 ms, to successfully pace the heart using
extra-cardiovascular electrodes without causing unacceptable pain
or discomfort to the patient. The cardiac electrical signals
received by IMD 14 are used for determining the patient's heart
rhythm and providing appropriate pacing therapy as needed, such as
bradycardia pacing or ATP. IMD 14 is configured as an ICD in this
example, capable of detecting shockable rhythms and delivering a
CV/DF shock therapy via defibrillation electrode 24 carried by lead
16. In other examples, IMD 14 may be configured as a pacemaker for
delivering low voltage pacing therapies without high voltage CV/DF
shock therapy capability. In this case, lead 16 may be carry pacing
and sensing electrodes, e.g., electrodes 28 and 30, without
incorporating a defibrillation electrode 24 or defibrillation
electrode 24 may be included and used as a return anode during
cardiac pacing using electrode 28 or 30 as a cathode.
Lead 16 includes a proximal end 27 that is connected to IMD 14 and
a distal portion 25 that carries electrodes 24, 28 and 30.
Electrode 24 is a defibrillation electrode that may be used in
combination with the conductive housing 15 of IMD 14 for delivering
high voltage CV/DF shocks. All or a portion of housing 15 of IMD 14
may be formed of a conductive material, such as titanium or
titanium alloy, and coupled to internal IMD circuitry to function
as an electrode, sometimes referred to as a "CAN electrode." A
shock vector pathway extends from defibrillation electrode 24 to
housing 15, through the ventricular myocardium. Defibrillation
electrode 24 is typically an elongated coil electrode having a
relatively higher surface area than electrodes 28 and 30 but may be
implemented as another type of electrode other than a coil
electrode.
Electrodes 28 and 30 are referred to herein as pacing and sensing
electrodes because they generally are used for delivering pacing
pulses and sensing cardiac electrical signals. An ECG signal may be
acquired using any combination of electrodes 28, 30 and housing 15.
For example, IMD 14 may sense cardiac electrical signals using a
sensing electrode vector between electrodes 28 and 30, a sensing
electrode vector between electrode 28 and housing 15 or a sensing
electrode vector between electrode 30 and housing 15 may be chosen.
In some examples, a sensing electrode vector may even include
defibrillation electrode 24, e.g., in conjunction with one or more
of electrodes 28, 30, or housing 15. IMD 14 may include more than
one sensing channel such that electrode sensing vectors may be
selected two at a time by IMD 14 for monitoring for a shockable
rhythm or a need for cardiac pacing.
Pacing pulses may be delivered using any combination of electrodes
24, 28, 30 and housing 15. The pacing electrode vector selected for
delivering pacing pulses may be selected based on pacing electrode
vector impedance measurements and capture threshold testing. For
example, a pacing electrode vector may be selected from among
electrodes 24, 28, 30 and housing 15 that has the lowest impedance
and/or the lowest pulse width that captures the heart for a
programmed pacing pulse amplitude. The pacing pulse amplitude may
be programmed to be below a threshold for pain and discomfort,
which may be based on individual patient testing and/or clinical
data.
While three electrodes 24, 28 and 30 are shown along lead 16, lead
16 may carry more or fewer electrodes in other examples. In the
example illustrated in FIG. 1, pacing and sensing electrodes 28 and
30 are separated from one another by defibrillation electrode 24.
In other words, sensing electrode 28 is located distal to
defibrillation electrode 24, and sensing electrode 30 is proximal
to defibrillation electrode 24. In various examples, electrodes 28
and 30 may be carried along lead 16 at other locations than those
shown but are generally positioned to acquire cardiac electrical
signals having acceptable cardiac signal strength for sensing
cardiac events, such as R-wave signals that occur upon
depolarization of the ventricles and for delivering low voltage
pacing pulses for successfully capturing the patient's heart 26.
Pacing pulses may be delivered using any combination of electrodes
24, 28, 30 and/or housing 15, e.g., using electrodes 28 and 30 in a
bipolar pair, using one of electrodes 28 or 30 paired with housing
15, using one of electrodes 28 or 30 paired with defibrillation
electrode 24, using both or electrodes 28 and 30 tied together as a
multi-polar cathode electrode paired with housing 15 or with
defibrillation electrode 24, and so on.
Two or more electrodes used for delivering low voltage pacing
pulses may be located at different locations along lead 16 in other
examples. For instance, an electrode configuration including two
pacing and sensing electrodes that are spaced apart along lead body
18 adjacent to each other without an intervening defibrillation
electrode 24 may be used for delivering low voltage pacing pulses
as disclosed herein. Two pacing and sensing electrodes may be
positioned adjacent to each other at spaced apart locations in
between two defibrillation electrodes as generally disclosed in
commonly-assigned U.S. patent application Ser. No. 14/519,436 and
U.S. patent application Ser. No. 14/695,255, both of which are
incorporated herein by reference in their entirety.
In other examples, an extra-cardiovascular lead may include
multiple defibrillation electrode segments, and multiple pacing and
sensing electrodes may be disposed between the defibrillation
electrode segments. The defibrillation electrode segments and
pacing and sensing electrodes, which may be ring electrodes, may be
carried by an undulating or zig-zagging distal portion of the lead
body as generally disclosed in provisionally-filed U.S. Pat.
Application No. 62/089,417, and may be utilized in conjunction with
the pacing techniques disclosed herein. U.S. Pat. Application No.
62/089,417 is also incorporated herein by reference in its
entirety. In still other examples, lead 16 may carry a single
pace/sense electrode to serve as a pacing cathode electrode with
housing 15 or with a defibrillation electrode 24 or any of the
defibrillation electrodes or defibrillation electrode segments
shown and described in the above-incorporated references, serving
as the return anode electrode.
In other examples, dedicated pacing electrodes and separate,
dedicated sensing electrodes may be carried by lead 16 or another
lead coupled to IMD 14. It is understood that one or more leads may
be coupled to IMD 14 for connecting at least one pacing and sensing
electrode to IMD 14 for monitoring cardiac electrical signals, and
delivering low voltage pacing pulses and at least one
defibrillation electrode and for delivering CV/DF shock therapy
when IMD 14 is configured as an ICD. Pacing therapies that may be
delivered by IMD 14 may include bradycardia pacing, ATP, CRT and/or
post-shock pacing for treating bradycardia or asystole after a
CV/DF shock.
Lead 16 is illustrated in FIG. 1 as being implanted at least
partially in a substernal location, e.g., between the heart 26 and
ribcage 32 or sternum 22. In one such configuration, the proximal
portion of lead 16 extends subcutaneously from IMD 14 (which is
implanted near a midaxillary line on the left side of patient 12)
toward sternum 22. At a location near xiphoid process 20, lead 16
bends or turns superiorly and distal portion 25 of lead 16, which
carries electrodes 24, 28 and 30, extends substernally, under or
below the sternum 22 in the anterior mediastinum 36.
FIG. 2 is a transverse view of patient 12 showing the distal
portion 25 of lead 16 extending substernally, e.g., at least
partially in or adjacent to the anterior mediastinum 36. Anterior
mediastinum 36 is bounded laterally by pleurae 39, posteriorly by
pericardium 38, and anteriorly by sternum 22. In some instances,
the anterior wall of anterior mediastinum 36 may also be formed by
the transversus thoracis and one or more costal cartilages.
Anterior mediastinum 36 includes a quantity of loose connective
tissue (such as areolar tissue), adipose tissue, some lymph
vessels, lymph glands, substernal musculature (e.g., transverse
thoracic muscle), branches of the internal thoracic artery, the
thymus gland, and the internal thoracic vein. In one example, the
distal portion of lead 16 extends along the posterior side of
sternum 22 substantially within the loose connective tissue and/or
substernal musculature of anterior mediastinum 36. Lead 16 may be
at least partially implanted in other extra-cardiovascular,
intrathoracic locations, e.g., along ribcage 32 or along or
adjacent to the perimeter of the pericardium 38 or within the
pleural cavity.
IMD 14 may also be implanted at other subcutaneous or submuscular
locations on patient 12, such as further posterior on the torso
toward the posterior axillary line, further anterior on the torso
toward the anterior axillary line, in a pectoral region, or at
other locations of patient 12. In instances in which IMD 14 is
implanted pectorally, lead 16 would follow a different path, e.g.,
across the upper chest area and inferior along sternum 22. When the
IMD 14 is implanted in the pectoral region, the system 10 may
include a second lead that extends along the left side of the
patient and includes a defibrillation electrode and/or one or more
pacing electrodes positioned along the left side of the patient to
function as an anode or cathode of a therapy delivery vector
including another electrode located anteriorly for delivering
electrical stimulation to heart 26 positioned there between.
In other examples, lead 16 may be implanted at other
extra-cardiovascular locations. For instance, lead 16 may be
implanted subcutaneously or submuscularly, between the skin and the
ribcage 32 or between the skin and sternum 22. Lead 16 extends
subcutaneously from IMD 14 toward xiphoid process 20 as shown in
FIG. 1, but instead of extending substernally, lead 16 may bend or
turn at a location near xiphoid process 20 and extend
subcutaneously or submuscularly superior, substantially parallel to
sternum 22. The distal portion 25 of lead 16 may be parallel over
sternum 22 or laterally offset from sternum 22, to the left or the
right. In other examples, the distal portion 25 of lead 16 may be
angled laterally away from sternum 22, either to the left or the
right, such that the distal portion 25 extends non-parallel to
sternum 22.
In another example, IMD 14 may be implanted subcutaneously outside
the ribcage 32 in an anterior medial location. Lead 16 may be
tunneled subcutaneously into a location adjacent to a portion of
the latissimus dorsi muscle of patient 12, from a medial implant
pocket of IMD 14 laterally and posterially to the patient's back to
a location opposite heart 26 such that the heart 26 is generally
disposed between the IMD 14 and electrodes 24, 28 and 30. The
techniques disclosed herein for generating low voltage pacing
pulses for pacing the heart using extra-cardiovascular electrodes
are not limited to a particular subcutaneous, submuscular,
supra-sternal, substernal or intra-thoracic location of the
extra-cardiovascular electrodes.
Referring again to FIG. 1, lead 16 includes an elongated lead body
18 that carries the electrodes 24, 28 and 30 and insulates one or
more elongated electrical conductors (not illustrated) that extend
from a respective electrode 24, 28 and 30 through the lead body 18
to a proximal connector (not shown) that is coupled to IMD 14 at
lead proximal end 27. Lead body 18 may be formed from a
non-conductive material, such as silicone, polyurethane,
fluoropolymers, or mixtures thereof or other appropriate materials,
and is shaped to form one or more lumens within which the one or
more conductors extend. The conductors are electrically coupled to
IMD circuitry, such as a therapy delivery module and an electrical
sensing module, via connections in IMD connector assembly 17 that
includes a connector bore for receiving the proximal connector of
lead 16 and associated electrical feedthroughs crossing IMD housing
15. The electrical conductors transmit electrical stimulation
therapy from a therapy delivery module within IMD 14 to one or more
of electrodes 24, 28, and 30, and transmit cardiac electrical
signals from one or more of electrodes 24, 28, and 30 to the
sensing module within IMD 14.
Housing 15 forms a hermetic seal that protects internal electronic
components of IMD 14. As indicated above, housing 15 may function
as a "CAN electrode" since the conductive housing or a portion
thereof may be electrically coupled to internal circuitry to be
used as an indifferent or ground electrode during cardiac signal
sensing or during electrical stimulation therapy delivery. As will
be described in further detail herein, housing 15 may enclose one
or more processors, memory devices, transmitters, receivers,
sensors, sensing circuitry, therapy circuitry and other appropriate
components.
The example IMD system 10 of FIG. 1 is illustrative in nature and
should not be considered limiting of the techniques described in
this disclosure. The techniques disclosed herein may be implemented
in numerous ICD or pacemakers operating with electrode
configurations that include extra-cardiovascular electrodes for
delivering cardiac pacing pulses. The IMD system 10 is referred to
as an extra-cardiovascular IMD system because lead 16 is a
non-transvenous lead, positioned outside the blood vessels, heart
26 and pericardium 38.
An external device 40 is shown in telemetric communication with IMD
14 by a communication link 42. External device 40 may include a
processor, display, user interface, telemetry unit and other
components for communicating with IMD 14 for transmitting and
receiving data via communication link 42. Communication link 42 may
be established between IMD 14 and external device 40 using a radio
frequency (RF) link such as BLUETOOTH.RTM., Wi-Fi, or Medical
Implant Communication Service (MICS) or other RF or communication
frequency bandwidth.
External device 40 may be embodied as a programmer used in a
hospital, clinic or physician's office to retrieve data from IMD 14
and to program operating parameters and algorithms in IMD 14 for
controlling IMD functions. External device 40 may be used to
program cardiac rhythm detection parameters and therapy control
parameters used by IMD 14. Data stored or acquired by IMD 14,
including physiological signals or associated data derived
therefrom, results of device diagnostics, and histories of detected
rhythm episodes and delivered therapies, may be retrieved from IMD
14 by external device 40 following an interrogation command.
External device 40 may alternatively be embodied as a home monitor
or hand-held device.
FIG. 3 is a schematic diagram of IMD 14 according to one example.
The electronic circuitry enclosed within housing 15 includes
software, firmware and hardware that cooperatively monitor one or
more cardiac electrical signals, determine when a pacing therapy is
necessary, and deliver prescribed pacing therapies as needed. When
IMD 14 is configured as an ICD as illustrated herein, the software,
firmware and hardware is also configured to determine when a CV/DF
shock is necessary and deliver prescribed CV/DF shock therapies.
IMD 14 may be coupled to a lead, such as lead 16 shown in FIG. 1,
carrying extra-cardiovascular electrodes 24, 28 and 30, for
delivering pacing therapies, CV/DF shock therapies and sensing
cardiac electrical signals.
IMD 14 includes a control module 80, memory 82, therapy delivery
module 84, electrical sensing module 86, telemetry module 88,
impedance measurement module 90 and an optional sensor module 92. A
power source 98 provides power to the circuitry of IMD 14,
including each of the modules 80, 82, 84, 86, 88, 90 and 92 as
needed. Power source 98 may include one or more energy storage
devices, such as one or more rechargeable or non-rechargeable
batteries. Power source 98 is coupled to low voltage (LV) and high
voltage (HV) charging circuits included in therapy delivery module
84 for charging LV and HV capacitors, respectively, included in
therapy delivery module 84 for generating therapeutic electrical
stimulation pulses.
The functional blocks shown in FIG. 3 represent functionality
included in IMD 14 and may include any discrete and/or integrated
electronic circuit components that implement analog and/or digital
circuits capable of producing the functions attributed to IMD 14
herein. For example, the modules may include analog circuits, e.g.,
amplification circuits, filtering circuits, and/or other signal
conditioning circuits. The modules may also include digital
circuits, e.g., analog-to-digital converters, digital signal
processors (DSPs), combinational or sequential logic circuits,
integrated circuits, application specific integrated circuits
(ASICs), memory devices, etc. As used herein, the term "module"
refers to an ASIC, an electronic circuit, a processor (shared,
dedicated, or group) and memory that execute one or more software
or firmware programs, a combinational logic circuit, state machine,
or other suitable components that provide the described
functionality. The particular form of software, hardware and/or
firmware employed to implement the functionality disclosed herein
will be determined primarily by the particular system architecture
employed in the IMD and by the particular detection and therapy
delivery methodologies employed by the IMD. Providing software,
hardware, and/or firmware to accomplish the described functionality
in the context of any modern IMD system, given the disclosure
herein, is within the abilities of one of skill in the art.
Memory 82 may include any volatile, non-volatile, magnetic, or
electrical non-transitory computer readable storage media, such as
a random access memory (RAM), read-only memory (ROM), non-volatile
RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash
memory, or any other memory device. Furthermore, memory 82 may
include non-transitory, computer-readable media storing
instructions that, when executed by one or more processing
circuits, cause control module 80 or other IMD modules to perform
various functions attributed to IMD 14 or those IMD modules. The
non-transitory computer readable media storing the instructions may
include any of the media listed above.
The functions attributed to the modules herein may be embodied as
one or more processors, hardware, firmware, software, or any
combination thereof. Depiction of different features as modules is
intended to highlight different functional aspects and does not
necessarily imply that such modules must be realized by separate
hardware, firmware or software components. Rather, functionality
associated with one or more modules may be performed by separate
hardware, firmware or software components, or integrated within
common hardware, firmware and/or software components. For example,
pacing therapy control operations performed by control module 80
may be implemented in a processor executing instructions stored in
memory 82.
Control module 80 communicates with therapy delivery module 84 and
electrical sensing module 86 for sensing cardiac electrical
activity, detecting cardiac rhythms, and controlling delivery of
cardiac electrical stimulation therapies in response to sensed
cardiac signals. Therapy delivery module 84 and electrical sensing
module 86 are electrically coupled to electrodes 24, 28, and 30
carried by lead 16 (shown in FIG. 1) and the housing 15, which may
function as a common or ground electrode.
Electrical sensing module 86 is selectively coupled to electrodes
28, 30 and housing 15 in order to monitor electrical activity of
the patient's heart. Electrical sensing module 86 may additionally
be selectively coupled to electrode 24. Sensing module 86 is
enabled to selectively monitor one or more sensing vectors selected
from the available electrodes 24, 28, 30 and 15. For example,
sensing module 86 may include switching circuitry for selecting
which of electrodes 24, 28, 30 and housing 15 are coupled to sense
amplifiers or other cardiac event detection circuitry included in
sensing module 86. Switching circuitry may include a switch array,
switch matrix, multiplexer, or any other type of switching device
suitable to selectively couple sense amplifiers to selected
electrodes. The cardiac event detection circuitry within electrical
sensing module 86 may include one or more sense amplifiers,
filters, rectifiers, threshold detectors, comparators,
analog-to-digital converters (ADCs), or other analog or digital
components.
In some examples, electrical sensing module 86 includes multiple
sensing channels for acquiring cardiac electrical signals from
multiple sensing electrode vectors selected from electrodes 24, 28,
30 and housing 15. Each sensing channel may be configured to
amplify, filter and rectify the cardiac electrical signal received
from selected electrodes coupled to the respective sensing channel
to improve the signal quality for sensing cardiac events, e.g.,
R-waves.
Each sensing channel includes cardiac event detection circuitry for
sensing cardiac events from the received cardiac electrical signal
developed across the selected electrodes 24, 28, 30 and/or 15.
Cardiac event sensing thresholds used by each sensing channel may
be automatically adjusted according to sensing control parameters,
which may be stored in memory 82. Each sensing channel senses a
cardiac event when the respectively received cardiac electrical
signal crosses the auto-adjusting cardiac event sensing
threshold.
Each time the received cardiac electrical signal crosses the
sensing threshold for a given channel, a cardiac sensed event
signal is produced and passed to control module 80. For example,
R-wave sensed event signals may be passed to control module 80 when
a received cardiac electrical signal crosses an R-wave sensing
threshold. Sensed event signals produced by electrical sensing
module 86 may be used by control module 80 for detecting a
shockable rhythm and/or for detecting a need for pacing. For
example, control module 80 may respond to sensed event signals by
setting pacing escape intervals for controlling the timing of
pacing pulses delivered by therapy delivery module 84. In addition
to the sensed cardiac event signals, electrical sensing module 86
may output a digitized ECG signal for use by control module 80 in
detecting/confirming tachycardia, e.g., via a morphology or wavelet
analysis.
Therapy delivery module 84 includes an LV therapy delivery module
85 for delivering low voltage pacing pulses using an
extra-cardiovascular pacing electrode vector selected from
electrodes 24, 28, 30 and housing 15. The LV therapy delivery
module includes an array of capacitors that are selectably
controlled by control module 80 to provide low voltage, long pulse
width pacing pulses having a truncated pulse amplitude at pulse
termination that is greater than a threshold amplitude. LV
capacitors included in the LV therapy delivery module 85 are
charged to a voltage according to a programmed pacing pulse
amplitude by an LV charging circuit (not shown in FIG. 3) included
in therapy delivery module 84. At an appropriate time, the LV
therapy delivery module 85 couples a selected capacitor
configuration to a pacing electrode vector to discharge the
capacitor configuration over a predetermined pacing pulse
width.
As described below, a pacing control module included in control
module 80 may be configured to receive a feedback signal from LV
therapy delivery module 85. The feedback signal indicates the
amplitude of the pacing pulse as it is decaying over the pacing
pulse width during capacitor discharge. If the pulse amplitude
falls to a threshold amplitude before the end of the pulse width,
the pacing control module may be configured to adjust the output
signal of the LV therapy delivery module 85 by enabling at least
one additional capacitor in the capacitor array of LV therapy
delivery module 85. By enabling an additional capacitor(s), the
voltage amplitude of the pacing pulse is maintained at or above a
minimum acceptable amplitude during the relatively long discharge
period defined by the pacing pulse width. The minimum acceptable
amplitude is the minimum amplitude at which capture and successful
pacing of the heart is highly probable.
Impedance measurement module 90 may be electrically coupled to the
available electrodes 24, 28 and 30 and housing 15 for performing
impedance measurements of a selected pacing electrode vector.
Control module 80 may control impedance measurement module 90 to
perform impedance measurements prior to pacing pulse delivery
and/or during a pacing pulse delivery. For example, control module
80 may pass a signal to impedance measurement module 90 to initiate
an impedance measurement for a selected pacing electrode vector.
Impedance measurement module 90 is configured to apply a drive or
excitation current across a selected pacing electrode vector and
determine the resulting voltage. The voltage signal may be used
directly as the impedance measurement or impedance may be
determined from the applied current and the measured voltage. The
impedance measurement is passed to control module 80 for use in
selecting a capacitor configuration for delivering pacing pulses.
The capacitor configuration selected based on the impedance
measurement may be the initial capacitor configuration used to
deliver a pacing pulse or an adjusted capacitor configuration used
to adjust the pacing pulse amplitude in real-time during a pacing
pulse. Capacitor configuration data is passed from control module
80 to LV therapy delivery module 85 for use in delivering pacing
pulses using the selected capacitor configuration as described in
greater detail below.
Therapy delivery module 84 may additionally include HV therapy
delivery module 83 including one or more HV output capacitors. When
a shockable rhythm is detected, the HV capacitors are charged to a
pre-programmed voltage level by a HV charging circuit, which may
include one or more transformers, switches, diodes, or the like.
Control module 80 applies a signal to trigger discharge of the HV
capacitors upon detecting a feedback signal from therapy delivery
module 84 that the HV capacitors have reached the voltage required
to deliver a programmed shock energy. In this way, control module
80 controls operation of the high voltage output circuit of therapy
delivery module 84 to deliver high energy CV/DF shocks using
defibrillation electrode 24 and housing 15. High energy CV/DF
shocks are generally on the order of at least 5 Joules and more
commonly on the order of 20 Joules or higher. In contrast, low
voltage pacing pulses delivered using extra-cardiovascular
electrodes may be on the order of 0.1 Joules or less, whereas
pacing pulses delivered using endocardial electrodes or epicardial
electrodes may be on the order of microJoules, e.g., 2 microJoules
to 5 microJoules for a typical pacing pulse that is 2V in
amplitude, 0.5 ms in pulse width and applied across a pacing
electrode vector impedance of 400 to 1,000 ohms.
Sensor module 92 may include additional sensors for monitoring the
patient and/or for controlling therapy delivery. For example,
sensor module 92 may include an activity sensor, a posture sensor,
a heart sound sensor, or other physiological sensor(s) for
monitoring the patient and making therapy delivery decisions. In
various examples, rate responsive pacing may be provided based on a
patient activity signal. The pacing rate delivered using
extra-cardiovascular electrodes may be increased according to an
increased metabolic demand of the patient as evidenced by the
patient activity signal. A decision to deliver ATP pulses (using
extra-cardiovascular electrodes, e.g., electrodes 24, 28, 30 and/or
housing 15 or other extra-cardiovascular electrode configurations
referred to herein) or shock therapy (using defibrillation
electrode 24 and housing 15) may be based in part on physiological
sensor signals in addition to the cardiac electrical signal.
Control parameters utilized by control module 80 may be programmed
into memory 82 via telemetry module 88. For example, the pacing
pulse width and pacing pulse amplitude may be programmable
parameters. Control module 80 may utilize the programmed pacing
pulse width and pacing pulse amplitude for controlling the
selection and charging of LV capacitors included in LV therapy
delivery module 85. Telemetry module 88 includes a transceiver and
antenna for communicating with external device 40 (shown in FIG. 1)
using RF communication as described above. Under the control of
control module 80, telemetry module 88 may receive downlink
telemetry from and send uplink telemetry to external device 40. In
some cases, telemetry module 88 may be used to transmit and receive
communication signals to/from another medical device implanted in
patient 12.
FIG. 4 is a schematic diagram of a pacing control module 102
included in control module 80 and the LV therapy delivery module 85
included in therapy delivery module 84. LV therapy delivery module
85 includes a capacitor selection and control module 104 and a
capacitor array 110 including multiple parallel capacitors C1
through Cn, where n is six in the example shown. Capacitor array
110 includes multiple switches S1 through Sn (S6 in the example
shown) that are controlled by capacitor selection and control
module 104 to selectively enable capacitors C1 through C6 for
pacing pulse delivery. Switches S1 through S6 each enable a
respective one of capacitors C1 through C6 when closed by coupling
the enabled capacitor to pacing pulse output signal line 130 when
switch 112 is also closed.
While six capacitors are shown, capacitor array 110 may include
more or fewer capacitors, which may depend on the requirements of
the particular pacing application, and available volume in the
housing 15. Capacitors C1 through C6 may be provided with a
capacitance of 20 microfarads in one example but capacitances
greater than or less than 20 microfarads may be used, e.g., 10
microfarads to 40 microfarads. Capacitors C1 through C6 may all
have the same capacitance values or different values to provide
different selectable effective capacitances for achieving various
RC time constants of the pacing discharge circuit and desired
ranges of pacing pulse widths for an expected range of pacing
electrode vector impedance.
A longer effective pulse width is possible when two or more of
capacitors C1 through C6 are selected together in parallel than
when any one is selected alone. The capacitors in parallel have an
effective capacitance equal to the sum of the parallel capacitances
for a total capacitance greater than any one of the capacitors
selected alone. The greater capacitance increases the RC time
constant for a given pacing electrode vector impedance. A higher
capacitance and RC time constant decreases the decay rate of the
pacing pulse and increases the maximum possible pacing pulse width
for a given programmed pulse amplitude and pacing electrode vector
impedance. The longer pulse width results in greater pulse energy
delivered across the pacing electrode vector for capturing the
heart. If the pulse decays too rapidly, the pulse amplitude at the
expiration of the pulse width may be too low to successfully
capture the heart. In order to achieve capture using a non-painful,
leading edge voltage amplitude of an extra-cardiovascular pacing
pulse, a relatively long pulse width, e.g. greater than 2.0 ms, may
be needed over which the decaying pulse amplitude is maintained
above a minimum threshold so that the total delivered pulse energy
is above the capture threshold of the heart.
In one example, C1 has a higher capacitance than capacitors C2
through C6 to provide a relatively long decay time when C1 is
selected alone for pacing pulse delivery. C2 through C6 may be
provided with lower capacitance values than C1 so that capacitor
configurations can be selected having higher effective capacitance
values at desired increments greater than the C1 capacitance.
Capacitor selection and control module 104 controls LV charging
circuit 114 to charge capacitor array 110 for supplying pacing
pulse energy. LV charging circuit 114 charges capacitor array 110
to a voltage level according to a programmed pacing pulse
amplitude. Power source 98 may provide regulated power to LV
charging circuit 114. LV charging circuit 114 may be controlled to
charge all capacitors C1-C6 or only selected ones of capacitors
C1-C6 to a voltage required for generating a pacing pulse having a
leading edge voltage amplitude at the programmed pulse amplitude.
LV charging circuit 114 includes a charge pump to charge the
parallel capacitors of array 110 when signaled by capacitor
selection and control module 104, which also closes switches S1
through S6 to enable charging of respective capacitors C1 through
C6. LV charging circuit 114 may include a voltmeter or other
indicator for providing a feedback signal to the charge pump during
charging and a comparator to determine when charging is complete,
e.g., when the charge reaches the programmed pulse amplitude.
LV charging circuit 114 will monitor and control the charging of
capacitors C1-C6 and may pass a control signal to capacitor
selection and control module 104 for controlling switch 112 to be
open while LV charging circuit 114 is charging to uncouple
capacitor array 110 from output signal line 130. LV charging
circuit 114 may pass a charge completion signal to capacitor
selection and control module 104 when the capacitor selection is
charged to a desired voltage. Switch 112 may be closed by capacitor
selection and control module 104 after charge completion and when
it is time to start pacing pulse delivery.
Capacitor selection and control module 104 receives control signals
and instructions from pacing control module 102 including capacitor
configuration data, pacing pulse timing data, and pacing pulse
amplitude and pulse width. In response to a signal from pacing
control module 102, capacitor selection and control module 104
enables a selected capacitor configuration of capacitor array 110,
by closing respective switches S1 to S6, and couples the selected
capacitor configuration to output signal line 130 via switch 112 to
discharge the selected capacitor configuration across a pacing
electrode vector coupled to output signal line 130. The capacitor
selection and control module 104 uncouples capacitor array 110 from
output signal line 130 by opening switch 112 at the expiration of a
programmed pacing pulse width. The pacing pulse is terminated when
switch 112 is opened.
In some examples, LV therapy delivery module 85 includes an
analog-to-digital converter (ADC) 106 for sampling the pacing pulse
amplitude in real-time and providing a digital feedback signal of
the sampled amplitude to pacing control module 102 on signal line
122. During the pacing pulse, pacing control module 102 enables ADC
106 to sample the pacing pulse output signal on output signal line
130 at a desired sampling rate, e.g., every 2 ms, throughout the
pacing pulse width. ADC 106 may be enabled to sample the pacing
pulse amplitude from the start of the pacing pulse when switch 112
is enabled (closed) until the end of the pacing pulse when switch
112 is disabled (opened). In other examples, ADC 106 may be enabled
at a predetermined time interval after the start of the pacing
pulse, e.g. after a first portion of the pacing pulse width.
The ADC 106 does not disrupt the pacing pulse but samples the pulse
amplitude and converts the analog pulse amplitude to a digital
representation for processing by pacing control module 102. Pacing
control module 102 monitors the sampled pacing pulse amplitude
received from ADC 106 during pacing pulse delivery by comparing the
sample points to a pre-determined amplitude threshold or to an
expected amplitude based on predicted values. In one example, the
sample points are compared to an amplitude threshold set as a
percentage of the programmed pacing pulse amplitude, e.g., 50% of
the programmed pacing pulse amplitude.
If the pacing pulse amplitude falls below the amplitude threshold,
the pacing control module 102 selects a second capacitor
configuration having an appropriate capacitance and stored energy
to compensate for the decayed charge of the initial capacitor
configuration. By enabling a second capacitor configuration that is
holding the charged voltage amplitude, the pacing pulse amplitude
is increased during pulse delivery, preserving the effective
longevity of the pacing pulse width and delivered energy required
for capturing of the heart. In this way, the capacitor selection
can be reconfigured during a single pacing pulse to increase the
pacing pulse voltage amplitude in the event of a faster than
anticipated decay rate. The capacitor selection and control module
104 is capable of managing the switches S1 to S6 to achieve
real-time capacitor reconfiguration during a pacing pulse. The
sampling interval, e.g., 2 ms or less, may be adjusted as needed to
enable real-time adjustment of the capacitor configuration from an
initial capacitor configuration to a second capacitor configuration
during the pacing pulse to maintain the pulse amplitude above a
minimum acceptable threshold.
In one example, the second capacitor configuration is selected by
passing a control signal to capacitor selection and control module
104 to enable at least one capacitor of capacitor array 110 that
was not included in the initial capacitor configuration. The
capacitor(s) of the second capacitor configuration restore a higher
pacing pulse amplitude to ensure the programmed pulse width and
required total pulse energy to capture the heart is achieved, even
under changing in vivo conditions such as changing impedance along
the pacing vector.
All or at least a portion of capacitors C1 through C6 are charged
prior to pacing pulse delivery. At least one capacitor more than
the number of capacitors being selected in the initial capacitor
configuration may be charged prior to pacing pulse delivery. In
response to a control signal from pacing control module 102,
capacitor selection and control module 104 reconfigures the initial
capacitor configuration to a second capacitor configuration that
includes at least one capacitor not used in the initial capacitor
configuration.
In an illustrative example, C1, C2 and C3 may be initially enabled
by capacitor selection and control module 104. Capacitors C1, C2,
and C3 are collectively discharged across the pacing electrode
vector via output signal line 130 to begin delivery of the pacing
pulse at the desired pulse amplitude. The output of capacitors C1,
C2, and C3 decays over time during capacitor discharge. If pacing
control module 102 determines that the sampled pulse amplitude has
reached or fallen below a pre-determined threshold during the
pacing pulse, pacing control module 102 passes a signal to
capacitor selection and control module 104 to couple at least one
new capacitor to pacing pulse output signal line 130. For example,
capacitor selection and control module 104 may enable capacitor C4
via switch S4. By adding a capacitor C4 to the previously-enabled,
discharging capacitors C1, C2 and C3 during pacing pulse delivery,
the pulse amplitude may be maintained above a minimum pulse
amplitude throughout the pacing pulse width. C1, C2 and C3 may be
disabled or switched off when C4 is enabled to ensure all the
energy from C4 is directed to the output signal line 130 via switch
112. In other examples, circuitry such as one or more diodes may be
included in capacitor array 110 to prevent charge distribution from
newly-enabled capacitor(s) to the partially-discharged initial
capacitor(s) and promote current flow produced by all discharging
capacitors to output line 130 so that all capacitors discharge
across the pacing electrode vector.
The added capacitor C4 in the second capacitor configuration may be
previously charged, prior to pacing pulse delivery. Frequent
sampling of the pulse amplitude may allow an amplitude threshold
crossing to be predicted and thereby allow charging of additional
capacitors on an as needed basis. Pacing control module 102 may
trigger capacitor selection and control module 104 to charge an
additional capacitor(s), C4 in this example, in preparation for
enabling a second capacitor configuration if needed.
Capacitor selection and control module 104 may control which of
capacitors C1 through C6 are re-charged after delivering a pacing
pulse. In some examples, all available capacitors C1 through C6 are
fully charged between pacing pulses and remain charged if not used
for pacing pulse delivery. The charge of any unused capacitors may
be topped off between pacing pulses. In other cases, the number of
capacitors beyond the initial capacitor configuration that are
charged and available for adding to the discharge circuit during
pacing pulse delivery may include all the remaining available
capacitors or a preset number of additional capacitors, e.g., one
to three additional capacitors. In other examples the number of
additional capacitors may depend on the pulse width. To illustrate,
for very long pulse widths, e.g., greater than 10 ms, all available
capacitors may be charged. For relatively short pulse widths, e.g.,
less than 5 ms, one additional capacitor may be charged. For
moderate pulse widths, e.g., from 5 ms to 10 ms, two additional
capacitors may be charged. Since the pulse amplitude may be more
likely to fall below a minimum threshold amplitude before the end
of the pacing pulse when longer pulse widths are used, a greater
number of additional capacitors may be charged to be available
during pacing pulse delivery. The number of additional capacitors
available may be limited by how many are being used in the initial
capacitor configuration.
FIG. 5 is a schematic diagram of pacing control module 102 included
in control module 80 and capable of accessing instructions stored
in memory 82. Pacing control module 102 may include a
microprocessor 140, data buffer 144, capacitor configuration module
146, ADC control module 148 and timing control module 150.
Microprocessor 140 may be configured to execute instructions stored
in memory 82 for selecting an initial capacitor configuration for
delivering a pacing pulse and for automatically adjusting the
capacitor configuration during the pacing pulse.
Microprocessor 140 provides capacitor configuration data to
capacitor configuration module 146 which passes the capacitor
configuration data to the capacitor selection and control module
104 of LV therapy delivery module 85 (FIG. 4). Microprocessor 140
may also pass instructions to ADC control module 148. ADC control
module 148 may be configured to control the sampling rate and
sampling time period(s) of ADC 106 (FIG. 4). Pulse amplitude sample
points are received by data buffer 144 from ADC 106 and passed to
microprocessor 140. Microprocessor 140 compares the sampled
amplitude values to an amplitude threshold. Based on this
comparison, microprocessor 140 determines if a capacitor
configuration change is required. If the sampled pulse amplitude is
at or below the threshold, a capacitor configuration adjustment is
needed to maintain the pulse amplitude above a minimum acceptable
amplitude throughout the programmed pulse width.
Microprocessor 140 passes new configuration data to capacitor
configuration module 146 in response to the sampled amplitude
falling to or below the threshold amplitude. Capacitor
configuration module 146 in turn passes the new configuration data
to capacitor selection and control module 104 of LV therapy
delivery module 85, e.g., on the next clock signal. The capacitor
configuration is now set until another reconfiguration occurs. In
this way, the pacing pulse output signal amplitude is adjusted by
changing the capacitor configuration during delivery of the pacing
pulse, i.e., before the pacing pulse width expires, in
real-time.
Data buffer 144 may receive impedance data from impedance
measurement module 90. Microprocessor 140 may retrieve the
impedance measurement data for use in determining an initial
capacitor configuration and/or in selecting an adjusted capacitor
configuration in response to the sampled pulse amplitude falling to
or below a threshold. A request for impedance data may be made
after an amplitude threshold crossing during pulse delivery to
assist in determining a capacitance required in the second
capacitor configuration to adjust the pacing pulse amplitude and
maintain it above a minimum amplitude. A new impedance measurement
request may additionally or alternatively be made between pacing
pulses based on the pulse amplitude reaching an amplitude threshold
during the previous pacing pulse.
Microprocessor 140 may determine the initial capacitor
configuration by computing the capacitance required to achieve an
RC time constant of the capacitor array and known impedance
measured for the selected pacing vector that enables maintaining
the pulse amplitude above a minimum amplitude for at least the
programmed pacing pulse width. Operations performed for determining
a capacitor configuration are described below in conjunction with
FIGS. 9 through 11.
Timing control module 150 receives pacing pulse timing data from
microprocessor 140, including starting time of a scheduled pacing
pulse and the pulse width. The time to start pacing pulse delivery
and the pulse width are passed to capacitor selection and control
module 104. Capacitor selection and control module 104 selects the
capacitor configuration according to the configuration data
received from capacitor configuration module 146 and couples the
selected capacitor configuration to the output signal line 130
according to a pacing pulse start time passed from timing control
module 150, e.g., based on the expiration of a pacing escape
interval or other inter-pulse interval. Capacitor selection and
control module 104 uncouples the selected capacitor configuration
from the output signal line 130 upon expiration of the pacing pulse
width received from timing control module 150.
FIG. 6 is a schematic diagram of capacitor selection and control
module 104 according to one example. Capacitor selection and
control module 104 includes a configuration control module 160,
configuration latches 162, a pulse width timer 164 and pacing pulse
enable/disable gate 166. Configuration control module 160 receives
a clock signal 154 and an input signal 156 from capacitor
configuration control module 146 of pacing control module 102 (FIG.
5). The input signal 156 includes capacitor configuration data
indicating the number of capacitors C1 through C6 that are to be
coupled to output signal line 130 for delivering the next pacing
pulse.
Configuration control module 160 clocks the capacitor configuration
data stored in buffers or other memory devices to configuration
latches 162, which store the configuration data until passed to the
S1-S6 switches of the capacitor array 110 (FIG. 4). In accordance
with the configuration data, configuration latches 162 set separate
signals that are passed to each of the respective switches S1-S6 to
selectively enable or disable each one of capacitors C1 through C6
for pacing pulse delivery. Disabled capacitors may be charged and
remain charged until needed for pacing pulse delivery but are not
coupled to output signal line 130 for pacing pulse delivery.
Disabled capacitors may be initially inactive during a pacing
pulse, i.e., not coupled to output signal line 130, but are ready
to be enabled for pacing pulse delivery if an adjusted capacitor
configuration is deemed necessary during the pacing pulse. Disabled
capacitors are enabled by being coupled to output signal line 130
via appropriate control of switches S1 through S6.
Configuration control module 160 receives initial capacitor
configuration data via input signal 156 that sets the configuration
data for selectively enabling or disabling each one of capacitors
C1 through C6 according to an initial capacitance requirement to
achieve an RC time constant that is longer than the pacing pulse
width or other predefined time interval threshold. The
configuration data is passed to configuration latches 162 which
enable the capacitor(s) included in the initial capacitor
configuration to be used for pacing pulse delivery.
Pulse width timer 164 receives clock signal 154 and input from
timing control module 150 (FIG. 5) on signal line 158. Pulse width
timer passes a timing control signal to pulse enable/disable gate
166. For example, upon expiration of a pacing escape interval,
timing control module 150 passes a signal to pulse width timer 164
to enable LV therapy delivery module 85 to start a pacing pulse.
Pulse enable/disable signal gate 166 outputs a signal on signal
line 168 to switch 112 (FIG. 4) to start the pacing pulse. Switch
112 is controlled by gate 166 to couple the selected capacitor
configuration to pacing pulse output signal line 130.
After the initial capacitor configuration is coupled to the pacing
pulse output signal line 130 via switch 112, configuration control
module 160 may receive new capacitor configuration data via signal
line 156 from pacing control module 102 if the sampled pacing pulse
amplitude falls to or below an amplitude threshold. As described
above, pacing control module 102 receives the sampled pacing pulse
output signal amplitude and compares the sampled amplitude to the
amplitude threshold. If the pacing pulse amplitude does not fall to
or below an amplitude threshold during the pulse width, the initial
capacitor configuration remains unchanged during the pacing pulse.
If the amplitude falls to or below the amplitude threshold during
the pacing pulse width, new capacitor configuration data is passed
to configuration control module 160 during the present pacing pulse
delivery. Configuration control module 160 passes new configuration
data to configuration latches 162 which causes at least one
capacitor to be enabled that was not included in the initial
capacitor configuration by coupling the capacitor in with the
initial capacitor configuration via a respective switch (one of
switches S1-S6). Capacitors selected for the initial configuration
may be deactivated (switched out of the discharge circuit by
uncoupling from output signal line 130) in the adjusted capacitor
configuration to prevent charge distribution to those capacitors
rather than to the pulse output signal line 130 in some cases. New
configuration data may be passed to configuration control module
160 and onto configuration latches 162 multiple times during a
given pacing pulse if the sampled pacing pulse amplitude decays to
or below an amplitude threshold more than once before the pulse
width expires due to the programmed pulse duration exceeding the
discharge capacity of the currently selected capacitor
configuration.
Upon expiration of the pacing pulse width, pulse width timer 164
passes a pulse termination signal to pulse enable/disable gate 166
that outputs a signal on control signal line 168 that terminates
the pacing pulse by disabling switch 112 to uncouple the selected
capacitor configuration from output signal line 130. Pacing control
module 102 passes capacitor configuration data to configuration
control module 160 for the next scheduled pacing pulse. The
capacitor configuration for the next scheduled pacing pulse may be
re-determined by microprocessor 140 based on a new pacing vector
impedance measurement or selected as the final capacitor
configuration upon expiration of the preceding pacing pulse. Prior
pacing pulse amplitude behavior, which may be influenced by pacing
vector impedance variations, may influence the capacitor
configuration selected for the next pacing pulse delivery.
FIG. 7A is a conceptual diagram of a pacing pulse 200 generated by
LV therapy delivery module 85. Pacing pulse 200 has an initial or
leading edge pulse amplitude 202 determined by the charge value of
the capacitor configuration, in accordance with the programmed
pacing pulse voltage amplitude. The leading edge pulse amplitude
may be, for example with no limitation intended, approximately 5 V
to 8 V. The pacing pulse leading edge 206 occurs when pulse
enable/disable gate 166 enables or closes switch 112 to
electrically couple a selected capacitor configuration of capacitor
array 110 to output signal line 130. The selected capacitor
configuration discharges through the impedance of the pacing
electrode vector during the predetermined pacing pulse width 210.
Upon expiration of the pacing pulse width 210, the pulse
enable/disable gate 166 disables or opens switch 112 to terminate
the pacing pulse at terminating edge 208. The pacing pulse has a
truncated pulse amplitude 204 at the expiration of the pacing pulse
width 210.
The amplitude of the pacing pulse 200 decays from the leading edge
amplitude 202 to the truncated amplitude 204 over the pacing pulse
width 210 as the selected capacitor configuration discharges
through the pacing electrode vector impedance according to an RC
time constant 212. The RC time constant 212, sometimes referred to
as "tau," is the product of the pacing electrode vector impedance
and the capacitance of the selected capacitor configuration. By
definition, the RC time constant is the time for a capacitor to
discharge through a resistance to approximately 36.8% of its
initial charge. A change in pacing vector impedance alters the
impedance in the RC time constant, altering the discharge rate of
the capacitor configuration, observed as the decay rate of the
pacing pulse amplitude. In some examples, the pacing control module
102 is configured to detect a decay rate that is occurring too
quickly for the pacing pulse width 210 by comparing sampled pulse
amplitude values to a threshold amplitude and responding by
adjusting the capacitor configuration to restore a higher pulse
amplitude, a slower decay rate or both.
Microprocessor 140 of pacing control module 102 is configured to
determine a capacitor configuration including one or more
capacitors of capacitor array 110 having an overall capacitance
that, with the measured impedance of the pacing electrode vector,
results in an RC time constant 212 that is greater than the
programmed pacing pulse width 210. In some examples, the capacitor
configuration is selected such that the truncated amplitude 204 is
expected to be greater than a predetermined percentage of the
leading edge amplitude 202, e.g., greater than 50% of the leading
edge amplitude 202. A threshold voltage requirement of the
truncated amplitude 204 may be defined in order to promote
successful capture of the myocardium prior to or upon termination
of the pacing pulse 200. When extra-cardiovascular electrodes are
used for delivering pacing pulses, the leading edge amplitude 202
may be kept relatively low, e.g., 8 Volts or less and below a pain
threshold of the patient. In order to deliver enough energy to
successfully capture the myocardium, a relatively long pacing pulse
width 210 may be required. If the RC time constant 212 of a
selected capacitor configuration is too short, the pacing pulse
amplitude may decay too quickly such that the truncated amplitude
204 is below a minimum threshold and the total pacing pulse energy
is inadequate to capture and pace the heart.
In order to promote successful capture, the pacing control module
102 may be configured to select the capacitor configuration so that
the RC time constant 212 is greater than a time interval threshold.
For example, the capacitor configuration may be selected to have a
capacitance resulting in an RC time constant 212 that is at least
greater than the programmed pacing pulse width 210 or a multiple
thereof or greater than a maximum programmable pacing pulse width
or a multiple thereof, e.g., twice or three times the maximum
programmable pacing pulse width. In one example, the pulse width
may be programmable between 1.5 ms and 20 ms. The capacitor
configuration is selected based on the measured impedance of the
pacing vector so that the resulting RC time constant 212 of the
discharge circuit (the selected capacitor configuration discharging
through the pacing electrode vector impedance) is longer than the
pacing pulse width 210 and the truncated amplitude 204 is greater
than an amplitude threshold at the expiration of the pacing pulse
200. By selecting a capacitor configuration having a capacitance
that results in an RC time constant meeting these criteria, the
voltage amplitude over the duration of the pacing pulse is
controlled within an acceptable voltage range and the delivered
pulse energy can be expected to successfully pace the heart.
FIG. 7B is a conceptual diagram of a pacing pulse 220 that may be
delivered by IMD 14 using a capacitor configuration that is
adjusted during the pacing pulse delivery. Pacing pulse 220 has an
initial or leading edge pulse amplitude 222 determined by the
charge value of the capacitor configuration, in accordance with the
programmed pacing pulse voltage amplitude. The pacing pulse leading
edge 226 occurs when pulse enable/disable gate 166 enables or
closes switch 112 to electrically couple a selected capacitor
configuration of capacitor array 110 to output signal line 130. The
selected capacitor configuration discharges through the impedance
of the pacing electrode vector with a decay rate 223 that may be
faster than expected causing the pacing pulse amplitude to fall
below an amplitude threshold 225 prior to expiration of the
programmed pacing pulse width 210.
Pacing control module 102 may monitor the sampled pulse amplitude,
and, if the pulse amplitude falls to the threshold 225, pacing
control module 102 passes a new capacitor configuration to
capacitor selection and control module 104. The adjusted capacitor
configuration is enabled at 227. Upon enabling the adjusted
capacitor configuration, the pulse amplitude is increased above the
threshold 225 and decays at a second decay rate 229 until the pulse
width 210 expires. The second decay rate 229 may be the same or
different than the first decay rate 223 depending on the relative
capacitances of the initial capacitor configuration and the
adjusted capacitor configuration. The increase in pulse amplitude
achieved by coupling at least one charged capacitor to the
discharge circuit during the pacing pulse 220 maintains the pacing
pulse amplitude within an acceptable voltage range, e.g., between
and including the programmed pulse amplitude (corresponding to
leading edge amplitude 222) and the amplitude threshold 225. The
adjusted capacitor configuration prevents the pulse amplitude from
falling below the amplitude threshold 225. The truncation amplitude
224 is greater than the amplitude threshold 225 at terminating edge
228 of pulse 220.
In some examples, threshold 225 is set greater than a minimum
acceptable pulse amplitude threshold so that the capacitor
configuration can be adjusted before the pulse amplitude reaches
the minimum acceptable threshold. For example, if the programmed
pulse amplitude is 5 V, a minimum acceptable threshold may be 2.5
V. The amplitude threshold 225 used for triggering a capacitor
configuration adjustment may be 2.75 V so that the pulse amplitude
is always maintained above the minimum acceptable threshold.
Extra-cardiovascular leads and pacing electrodes may be subjected
to body motion resulting in shifting of the pacing electrodes and
changes in the pacing electrode vector impedance. Acute or chronic
changes in the impedance of a pacing electrode vector will be
accounted for in determining a capacitor configuration for a given
measured impedance. In some cases, pacing electrode vector
impedance may change during a pacing pulse. When pacing control
module 102 is configured to monitor the pulse amplitude in real
time, the pacing control module 102 can respond to a change in
pacing electrode vector impedance during a pacing pulse by
adjusting the capacitor configuration.
FIG. 7C is a conceptual diagram of one example of a pacing pulse
230 that may be delivered by IMD 14 using an adjusted capacitor
configuration during the pacing pulse when a pacing electrode
vector impedance change occurs. Pacing pulse 230 has a pulse
amplitude 232 at the leading edge 236 that is determined by the
charge value of the initial capacitor configuration that is
selected and based on the programmed pacing pulse amplitude. The
pacing pulse decays from the leading edge amplitude 232 at an
initial decay rate 233a according to the RC time constant of the
discharge circuit. The RC time constant is a function of the
capacitance of the initial capacitor configuration and the
impedance of the pacing electrode vector. At time 231, during the
pacing pulse 230, a change in pacing vector impedance occurs, e.g.,
due to a shift in electrode location or other factors. A decrease
in impedance causes the pulse to decay at a faster decay rate 233b
after time 231 in this example. The faster decay rate 233b causes
the pulse amplitude to reach amplitude threshold 235 prior to the
expiration of pulse width 210.
In this example, pacing control module 102 is configured to monitor
the pulse amplitude samples received from ADC 106 in real-time and
determine if the pulse amplitude reaches the amplitude threshold
235 during the pacing pulse. As described previously, the threshold
235 may be set as a percentage of programmed pulse amplitude and
may define a minimum acceptable voltage amplitude or be greater
than the minimum acceptable voltage amplitude so that the pulse
amplitude can be adjusted in real time to be maintained within an
acceptable voltage range.
Upon detecting that the pulse amplitude has reached the amplitude
threshold 235, pacing control module 102 passes adjusted capacitor
configuration data to capacitor selection and control module 104.
Capacitor selection and control module 104 enables the new
capacitor configuration to begin discharging across the pacing
vector resulting in a step increase 237 of the pacing pulse
amplitude. The step increase 237 is achieved by switching in at
least one fully charged capacitor to begin discharging during the
pacing pulse 230. Capacitors included in the initial capacitor
configuration may be disabled or may remain enabled in the adjusted
capacitor configuration.
The pulse 230 decays at a third decay rate 239 after the step
increase in amplitude. The third decay rate 239 will depend on the
capacitance of the adjusted capacitor configuration and the
impedance of the pacing electrode vector. In this example, the
capacitance of the adjusted capacitor configuration may be the same
as the initial capacitor configuration such that decay rate 239 is
approximately the same as the second decay rate 233b after the
pacing vector impedance change at 231. The step increase 237 in
pulse amplitude, however, maintains the pulse amplitude within an
acceptable range such that the truncated amplitude 234 at
terminating edge 238 is still greater than the amplitude threshold
235. In other examples, the adjusted capacitor configuration may be
selected to have a higher capacitance than the initial capacitor
configuration to slow the decay rate 239 after the decrease in
pacing electrode vector impedance.
FIG. 8 is a conceptual diagram of a look-up table 240 stored in
memory 82 accessible by pacing control module 102. Various
capacitor configurations may be stored in memory 82, e.g., in the
form of a look-up table, for different values of pacing electrode
vector impedance. The look-up table 240 includes multiple impedance
ranges 242 (listed in ohms), and a capacitor configuration 244
stored for each impedance range. Microprocessor 140 may be
configured to fetch an impedance measurement from data buffer 144
(FIG. 5) and compare the impedance measurement to the multiple
impedance ranges 242 included in the look-up table 240. When the
range of impedance is identified that includes or matches the
impedance measurement, the microprocessor 140 selects the capacitor
configuration that is stored for the matching impedance range.
As seen in the example of FIG. 8, the capacitor configurations 244
range from a capacitor configuration including all six capacitors,
C1-C6, of capacitor array 110 when the measured impedance is
between 100 and 249 ohms, inclusive, to a capacitor configuration
including only one capacitor, C1, when the measured impedance is
between 1600 and 2000 ohms, inclusive.
Using the example of capacitors C1-C6 each being 20 microfarad
capacitors, the RC time constant for the first capacitor
configuration C1-C6 is approximately 12 ms to 30 ms, depending on
the measured impedance in the first range of 100 ohms to 240 ohms.
The RC time constant is between approximately 25 ms and 50 ms for
the second capacitor configuration C1-C5, between approximately 40
and 75 ms for the C1-C4 configuration, between approximately 45 and
72 ms for the C1-C3 configuration, between approximately 48 and 64
ms for the C1-C2 configuration and between approximately 32 and 40
ms for the C1 configuration. Accordingly, for a pulse width of 10
ms, the RC time constant for a given impedance measurement and its
associated capacitor configuration is greater than the 10 ms pulse
width for all possible capacitor configurations. If the maximum
programmable pulse width is as high as 20 ms, all capacitor
configurations have an RC time constant greater than the maximum
programmable pulse width with the exception of the case when the
pacing vector impedance approaches the minimum listed impedance of
100 ohms.
A pacing vector impedance of 100 ohms is relatively low, but may
occur in some instances. For pacing pulse widths of 10 ms or
greater, the leading edge pulse amplitude may be required to be
relatively higher in order to deliver adequate energy prior to
expiration of the pacing pulse. For a shorter pulse width, e.g., 5
ms, the truncated amplitude is 3.29 V when the leading edge
amplitude is 5 V and the capacitor configuration C1-C6 is used. The
truncated amplitude is greater than 50% of the leading edge
amplitude.
In other examples, a stored look-up table such as table 240 is not
required. Microprocessor 140 may compute the capacitance required
to obtain an RC time constant greater than a threshold time
interval for the known, measured impedance. Microprocessor 140 may
then select the number of capacitors C1-C6 required to achieve an
overall capacitance that is equal to or greater than the computed,
required capacitance.
In the example look-up table 240 of FIG. 8, C1 is the default
capacitor enabled for all pacing pulses. Capacitors C2 through C6
may be added to a capacitor configuration as needed in a Cn+1
order, e.g., the configuration of C1-C2 may be selected if two
capacitors are needed, the configuration C1-C2-C3 may be selected
if three capacitors are needed and so on up to C1 through C6 if all
capacitors are included in the selected capacitor
configuration.
In other examples, when additional capacitors are needed, capacitor
selection and control module 104 may cycle through the remaining
capacitors from one pacing pulse to the next. For example, C1 may
remain the default capacitor but be paired with a different second
capacitor for successive pacing pulses when two capacitors are
needed, e.g., C1 and C2 for one pacing pulse, then C1 and C3 for
the next pacing pulse, then C1 and C4, and so on. In other
examples, capacitor selection and control module 104 may cycle the
capacitors selected for generating pacing pulses for a given
capacitance requirement of the capacitor configuration. For
example, if the selected capacitor configuration includes three
capacitors, capacitors C1, C2 and C3 may be enabled for delivering
one pacing pulse and capacitors C4, C5 and C6 may be used for
delivering the next pacing pulse such that the capacitors are
charged and enabled for pulse delivery in an alternating manner. It
is to be understood that practice of the techniques disclosed
herein are not limited to a particular order or sequence of
selecting capacitors included in a capacitor configuration. Rather,
the capacitors of array 110 may be selected in any combination in
order to achieve a required capacitance to achieve a pacing pulse
having the programmed pulse width and truncated voltage amplitude
that successfully captures the heart with a high degree of
confidence. By cycling or alternating between capacitors of array
110 when not all capacitors are required may allow the LV therapy
module 104 to maintain charge on capacitors between pacing pulses,
particularly when the pacing rate is relatively fast (shorter
escape interval), such as during ATP or rate responsive pacing at a
higher rate. If capacitors are charged between pulses to maintain a
charge, topping off pacing charge can be performed quickly. If more
complete charging is needed, cycling between capacitor
configurations allows one capacitor configuration to be charged for
every other pacing pulse while a different capacitor configuration
is delivering the intervening pacing pulses.
FIG. 9 is a flow chart of a method performed by IMD 14 for
delivering a pacing pulse according to one example. At block 252,
the control module 80 controls impedance measurement module 90 to
measure the impedance of a selected pacing electrode vector, e.g.
from electrode 28 to housing 15, from electrode 30 to housing 15,
or between electrodes 28 and 30 of FIGS. 1 and 2. At block 254,
microprocessor 140 of pacing control module 102 receives the
impedance measurement and determines a capacitor configuration for
delivering a pacing pulse across the measured impedance of the
selected pacing vector for the desired pulse width.
The capacitor configuration may be determined from a look-up table
stored in memory 82 as described in conjunction with FIG. 8.
Alternatively, the capacitor configuration may be determined by
computing the total capacitance required to achieve an RC time
constant that is greater than a threshold time interval for the
measured impedance of the selected pacing vector. The threshold
time interval may be based on the programmed pulse width and/or a
minimum amplitude at the terminating pulse edge. The capacitor
configuration is then selected at block 256 as the number of
capacitors required to meet or exceed the required total
capacitance and by identifying the capacitors of capacitor array
110 that are to be enabled in the initial capacitor
configuration.
In some examples, the capacitor configuration is determined to
achieve an RC time constant that is greater than the delivered
pulse width so that the truncated pulse amplitude at the expiration
of the delivered pulse width is greater than a minimum threshold.
The minimum threshold at the expiration of the pulse width may be a
predetermined percentage of the leading edge pulse amplitude, e.g.,
50% of the leading edge pulse amplitude.
To illustrate, if the pacing pulse width is programmed to 10 ms,
the measured impedance of the pacing electrode vector is 500 ohms,
and the capacitor array 110 includes six 20 microfarad capacitors,
the capacitor configuration may be determined at block 254 to
include three capacitors having a combined capacitance of 60
microfarads. The resulting time constant is 30 ms, greater than the
programmed pulse width of 10 ms. If the pacing pulse amplitude is
programmed to 5.0 V at the leading edge of the pacing pulse, the
truncated voltage amplitude at the expiration of the 10 ms pulse
width is expected to be approximately 3.58 V, greater than at least
50% of the leading edge voltage amplitude.
At block 256, the capacitor selection and control module 104
enables the selected capacitor configuration, e.g., by enabling or
disabling switches S1 through S6 as required to enable the
capacitors selected from C1 through C6 of capacitor array 110. The
selected capacitors are charged at block 258 and the pulse width
timer 164 is set at block 260. The pacing pulse is delivered at
block 262 by enabling switch 112 to couple the selected capacitor
configuration to output signal line 130 to allow the capacitor
configuration to discharge across the pacing electrode vector. When
the pulse width timer 164 expires, switch 112 is disabled or opened
to terminate the pacing pulse and uncouple the capacitor
configuration from output signal line 130 at block 264.
In flow chart 250, capacitor charging is performed after selection
of the individual capacitors of capacitor array 110 for use in the
determined capacitor configuration. In some examples, only the
capacitors included in the initial capacitor configuration are
charged for pacing pulse delivery. In other examples, at least one
additional capacitor is charged to be available to be added to the
initial capacitor configuration if the pacing pulse amplitude falls
to or below a threshold. In still other examples, all capacitors of
capacitor array 110 are charged prior to, during or after
determination of the capacitor configuration to be used for the
next pacing pulse. Only the capacitors selected according to the
initial capacitor configuration are coupled to the output signal
line 130 for initiating the pacing pulse. Any capacitors not
included in the initial capacitor configuration may remain charged
and available for selection in a second capacitor configuration
selected in response to the pulse amplitude falling below an
amplitude threshold prior to pulse width expiration or for
selection in an initial capacitor configuration for delivering a
future pacing pulse.
After pacing pulse delivery, the process shown in FIG. 9 returns to
block 252 to repeat the impedance measurement for the pacing
electrode vector. In some examples, the impedance measurement is
performed prior to each pacing pulse so that the initial capacitor
configuration is set in response to an impedance measurement for
each pacing pulse. In other examples, after setting the initial
capacitor configuration for the first pulse after an impedance
measurement, the same capacitor configuration may be used for
subsequent pacing pulses until a capacitor configuration adjustment
is required or until another impedance measurement occurs, e.g., on
a scheduled basis.
FIG. 10 is a flow chart 270 that includes operations that may be
performed in block 262 of FIG. 9 for delivering a pacing pulse.
Such operations include pacing pulse initiation and termination and
amplitude sampling during pulse delivery. The method of flow chart
270 may also include post-pulse delivery assessment of the
capacitor configuration used to deliver the pulse to guide
selection of the next initial capacitor configuration for
delivering the next pacing pulse. At block 271, the pacing pulse is
initiated by coupling the selected capacitor configuration to the
output signal line 130 at the appropriate time as controlled by
pulse width timer 164. The pacing pulse has a leading edge voltage
amplitude in accordance with the programmed pulse amplitude.
At block 272, the pacing control module 102 enables the ADC 106 to
begin sampling the pacing pulse amplitude at a predetermined
sampling rate, e.g., 2 milliseconds. Sampling occurs concurrently
with pulse delivery to provide real-time detection of an amplitude
threshold crossing. In one example, the ADC 106 is enabled at the
leading edge of the pacing pulse and the pacing pulse amplitude is
sampled throughout the pacing pulse from the leading edge or the
first sampling interval thereafter until the terminating edge or at
the last sampling interval preceding the terminating edge. In other
examples, the ADC 106 may be enabled to begin sampling after a time
delay after the leading edge, e.g. after a portion of the pulse
width. For instance, ADC 106 may be enabled to begin sampling
half-way through the pacing pulse width and continue sampling the
pacing pulse amplitude until the terminating edge or the last
sampling interval preceding the terminating edge. In still other
examples, the sampling interval may be variable. A longer sampling
interval may be used initially and shorten as the terminating edge
is approached.
In some cases, the pulse amplitude may be sampled at a mid-point of
the pacing pulse width and if the pulse amplitude is at least an
expected amplitude or greater, no further sampling is performed
during the pacing pulse. Using the example given above, for a 5V
pulse having a 10 ms pulse width that is expected to have a
truncated amplitude of 3.58 V at 10 ms, if the amplitude is sampled
at 5 ms and is at least an expected amplitude based on predictive
modeling, e.g., 4.2 V, no further sampling is needed. A reasonable
confidence exists that the truncated amplitude will remain above
the targeted minimum voltage, e.g., 50% of the leading edge voltage
or 2.5 V in this example. If the sampled amplitude is less than an
expected midpoint amplitude, however, ADC 106 may be enabled to
sample the pacing pulse at a predetermined sampling rate, e.g.,
every 2 ms or more often, for the remainder of the pacing pulse
width.
At block 274 the microprocessor 140 fetches the sampled signal
point from data buffer 144 and compares the sampled signal point to
an amplitude threshold at block 276. The amplitude threshold used
for the comparison at block 276 may be defined as a minimum
threshold, e.g., 50% or another percentage of the leading edge
amplitude (or programmed pulse amplitude). As suggested above, the
amplitude threshold may be a higher threshold used for comparison
to sample points acquired earlier in the pacing pulse width. For
example, at the midpoint of the pacing pulse width, the sample
point may be compared to a higher threshold, e.g., 85% of the
programmed pulse amplitude. From the midpoint to the terminating
edge of the pulse width, the sampled amplitude may be compared to a
lower threshold, e.g., 50% of the programmed pulse amplitude. In
other examples, the threshold may be a function of the expired time
of the pacing pulse width and based on an expected amplitude
according to the RC time constant of the capacitor configuration
and measured impedance of the pacing vector.
If the sampled pulse amplitude is less than the amplitude threshold
at block 276, the capacitor configuration may be adjusted at block
280. As described above, in some cases, if the pulse amplitude is
below a first threshold during a first portion of the pacing pulse
width, the ADC 106 may be enabled to sample more frequently during
a second portion of the pacing pulse width during which a second
lower threshold is used for the comparison at block 276. When the
sampled amplitude falls below the second lower threshold, the
capacitor configuration adjustment may be made at block 280. In
other examples, the capacitor configuration adjustment may be made
in response to the sampled amplitude falling to or below the first
higher threshold during the first portion of the pacing pulse if
detected first and otherwise in response to the sampled amplitude
falling to or below the second lower threshold during the second
portion of the pacing pulse.
In some examples, a limited number of capacitor configuration
adjustments may be performed. For instance, after the capacitor
configuration has been adjusted the first time during a pacing
pulse at block 280, no further adjustments of the capacitor
configuration are made. The ADC 106 may or may not continue to
sample the pulse amplitude during the remainder of the pacing pulse
after the pacing configuration has been changed. A maximum number
of capacitor configuration adjustments greater than one may be
allowed, e.g., up to two, three or more adjustments, in which case
the ADC 106 continues to sample the pulse amplitude for comparison
to the amplitude threshold at block 276 after the first adjustment.
If the maximum number of adjustments has been reached as determined
at block 277, the process may advance to block 278 to wait for the
pulse width timer 164 to expire. If the maximum number of capacitor
configuration adjustments has not been reached at block 277, and
the pulse width timer has not expired (block 279), the capacitor
configuration is adjusted at block 280.
After adjusting the capacitor configuration at block 280, ADC 106
may continue sampling the pacing pulse signal at block 274 and
passing signal sample points to data buffer 144 which may store the
data on a first-in, first-out basis. If the pacing pulse amplitude
falls below a threshold again before the pulse width timer expires,
the capacitor configuration may be adjusted again prior to
termination of the pacing pulse if the maximum number of
adjustments has not been reached.
The ADC 106 may be controlled to sample the pulse amplitude at a
regular sampling interval from the onset of the pulse or a
predetermined time interval after onset of the pulse until
expiration of the pulse width. In other examples, the ADC 106 may
be controlled to be disabled prior to completion of the pulse
delivery if the pulse amplitude has been sustained at an acceptable
level up to a predetermined portion of the pulse width. The time
that the ADC 106 is disabled may be based on sampling rate, the
programmed pulse width, the expired portion of the pulse width and
the pulse amplitude stability during the pulse. "Pulse amplitude
stability" refers to the amplitude remaining within a "constant"
voltage range as defined for a particular application. The pulse
amplitude may decay over the pulse width but remain within a
specified "constant" voltage range, e.g., between amplitude
threshold and the programmed pulse amplitude. If not, pulse
amplitude sampling continues and the capacitor configuration is
adjusted as needed. In some examples, all or some of the sample
points obtained during the pacing pulse may be passed to memory 82
to be stored for transmission to external device 40 for reviewing
and analyzing pacing performance of IMD 14.
If ADC 106 continues to sample the pulse amplitude after the
capacitor configuration adjustment while waiting for the pulse
width to expire, the additional amplitude data may be used by
microprocessor 140 for selecting future initial capacitor
configurations for subsequent pacing pulses and/or for selecting
adjusted capacitor configurations during subsequent pacing
pulses.
Upon expiration of the pulse width timer at block 278, the pacing
pulse is terminated at block 282. By adjusting the capacitor
configuration during the pacing pulse in response to the pulse
amplitude falling to or below an amplitude threshold, the truncated
amplitude may be maintained above a minimum threshold amplitude.
The truncated amplitude may be sampled to verify that the capacitor
configuration adjustment successfully maintained the truncated
amplitude above the minimum threshold amplitude. If the truncated
pulse amplitude reaches the minimum threshold amplitude at
truncation as determined at block 284, microprocessor 140 may
adjust the initial capacitor configuration used on a subsequent
pacing pulse at block 286 to maintain the pulse amplitude a safety
margin above the threshold amplitude at truncation of the next
pulse.
In some examples, pacing control module 102 may be configured to
adjust an initial capacitor configuration based on the sampled
pulse amplitude being greater than an expected maximum threshold.
For example, if the truncated amplitude is greater than an expected
maximum threshold, as determined at block 284, the capacitance may
be higher than necessary for the pacing vector impedance. The
initial capacitor configuration may be adjusted for the next pacing
pulse by eliminating one capacitor from the configuration at block
286. In some examples, the adjustment of the initial capacitor
configuration at block 286 includes repeating an impedance
measurement of the pacing vector to re-determine the required
capacitance of the capacitor configuration. If the truncated
amplitude is less than or equal to an expected maximum threshold
and greater than the minimum threshold, the initial capacitor
configuration may remain unchanged at block 288 or be set to the
final capacitor configuration that was used to deliver the pacing
pulse.
FIG. 11 is a flow chart 300 of a pacing control method performed by
IMD 14 according to another example. A pacing pulse is started at
block 302. The pacing control module 102 starts a pulse width timer
and controls the LV therapy delivery module 85 to start discharging
an initial capacitor configuration across an extra-cardiovascular
pacing electrode vector. The initial capacitor configuration may be
a default capacitor configuration, a previously selected capacitor
configuration, or set based on a pacing electrode vector impedance
measurement.
At block 304, microprocessor 140 determines that the pulse
amplitude has fallen to or below an amplitude threshold after a
first portion of the pulse width, such that a capacitor
configuration adjustment is needed during the pacing pulse. In
response to the pulse amplitude falling to or below the amplitude
threshold, the pacing control module 102 passes a new capacitor
configuration to capacitor selection and control module 104. At
block 306, capacitor selection and control module 104 adds one or
more capacitors to the initial capacitor configuration according to
configuration data received from pacing control module 102 to
produce a second capacitor configuration different than the initial
capacitor configuration. The second capacitor configuration
includes at least one capacitor not included in the initial
capacitor configuration but may have been previously charged, e.g.,
when the initial capacitor configuration was charged. If not
previously charged, the capacitor selection and control module 104
enables charging of the one or more capacitors added in the second
capacitor configuration and upon charge completion couples the
added capacitor(s) to the output signal line by enabling the
appropriate switch(es), e.g., one or more of S2 through S6 of FIG.
4 assuming at least C1 is included in the initial capacitor
configuration. The capacitors included in the initial capacitor
configuration may remain enabled and coupled to output signal line
130 or may be disabled upon adding the one or more capacitors of
the second capacitor configuration to the discharge circuit at
block 306.
The LV therapy delivery module 85 continues delivering the pacing
pulse using the second capacitor configuration at block 308, until
the pulse width expires. After enabling the second capacitor
configuration, microprocessor 140 may fetch pulse amplitude sample
point(s) during a second portion of the pacing pulse for comparison
to a minimum threshold at block 310. As long as the sampled pulse
amplitude remains greater than the minimum threshold, as determined
at block 312, the capacitor configuration adjustment is deemed
adequate for maintaining the truncated amplitude of the pacing
pulse above a minimum threshold. If the pulse amplitude falls below
the minimum threshold at block 312, however, an impedance
measurement trigger signal may be generated by microprocessor 140
at block 314. Control module 80 may control impedance measurement
module 90 to perform an impedance measurement of the pacing
electrode vector prior to the next pacing pulse.
At block 316, pacing control module 102 sets the initial capacitor
configuration for the next pacing pulse. If an impedance
measurement was triggered at block 314, the pacing control module
102 sets the initial capacitor configuration for the next pacing
pulse based on the triggered impedance measurement. The capacitance
required to achieve an RC time constant longer than a threshold
time interval may be determined using the triggered impedance
measurement. The new initial capacitor configuration is selected
according to the determined capacitance. Alternatively, the new
initial capacitor configuration may be retrieved from a look-up
table in memory 82 by matching the measured impedance to a stored
impedance range.
If an impedance measurement was not triggered at block 314, the
initial capacitor configuration set at block 316 may be set to the
same initial capacitor configuration as used on the currently
delivered pacing pulse or the adjusted capacitor configuration
which was used for delivering the pacing pulse during a second
portion of the pulse width. In this way, an impedance measurement
may be performed only when triggered in response to the pacing
pulse amplitude falling below a minimum threshold after a capacitor
configuration adjustment. In various examples, the impedance
measurement may be triggered when the capacitor configuration has
been adjusted a predetermined number of times, e.g., a single time,
two times, or other threshold number of times during a single
pacing pulse.
Thus, a method and apparatus for delivering pacing pulses using
extra-cardiovascular electrodes have been presented in the
foregoing description with reference to specific embodiments. In
other examples, various methods described herein may include steps
performed in a different order or combination than the illustrative
examples shown and described herein. It is appreciated that various
modifications to the referenced embodiments may be made without
departing from the scope of the disclosure and the following
claims.
* * * * *